The present invention relates to a method of compressing a dynamic range for a radiation image. The invention particularly relates to a technology for improving a method of compressing a dynamic range for a radiation image, the method allowing to process an original image signal and to obtain an image signal carrying an image having a dynamic range narrower than that of the original image.
Heretofore, there has been known, for example, a technology such as that disclosed in Japanese Patent Publication Open to Public Inspection No. 222577/1991 (hereinafter referred to as Japanese Patent O.P.I. Publication) as a method for compressing a density region while keeping a fine structure in an image region to be suitable for observation, in a radiation image.
In a compression method disclosed in Japanese Patent O.P.I. Publication No. 222577/1991 mentioned above, unsharp mask signal (blurred signal) Sus corresponding to each pixel point is obtained by averaging original image signal Sorg in a predetermined mask region including the pixel point, and processed image signal Sproc wherein a dynamic range is compressed is obtained through an expression EQU Sproc=Sorg+f(Sus)
when f(Sus) represents a function that monotonically decreases as the unsharp mask signal Sus increases.
Inventors of the present invention found that it is preferable to make the mask size for the unsharp mask signal Sus large when obtaining sufficient effects of compression while keeping a fine structure to be suitable for observation, in the above-mentioned method of compressing a dynamic range wherein unsharp mask signal Sus is used.
On the other hand, the present inventors found simultaneously the problem that when a mask size is large, the sharp cut of an edge of an unsharp image is deteriorated at a portion where signal values change sharply, and thereby unwelcome compression is carried out in the vicinity of a border between an region requiring compression and that requiring no compression. For example, there has been a problem that unwelcome compression is carried out in the vicinity of a border between a directly exposed region representing a high density region and an object region because of a big difference of signals between them, resulting in an occurrence of an artifact (a fabricated image).
In the case of simultaneous compression for both the low-density region and high-density region, when compression is carried out for the both density region based on the same compression rate, there has been a problem that the compression rate established to be suitable for the region of one side is hoe necessarily the best for the region of the other side.
In the dynamic range compression such as that mentioned above, it is an object to obtain the contrast characteristics which are optimum for image reading on the targeted region in the visualized state such a hard copy obtained finally. In the conventional dynamic range compression method, however, the following problems have been caused because of a determination of a degree of correction made based on inputted digital image signals (original image signals).
Namely, even when a certain correction function (for example, a linear function) is established for an inputted digital signal value, a correction degree of dynamic range compression is sometimes changed (the linear function disappears) when visualized finally, depending on gradation characteristics of a recording material or the like used for obtaining a hard copy, thus it has been impossible to provide stably the dynamic range compression processing that is optimum on the final state of indication (on a hard copy).
Furthermore, in the conventional dynamic range compression method wherein a dynamic range is compressed at a constant compression rate regardless of the dynamic range of an object, there has been a possibility of occurrence of a problem that excessive compression is caused in compression processing for a low-density area in a chest image of a person who is thinner than an average person, for example, and thereby density on a mediastinum region increases to lower the diagnostic power, while in the case of a fat person on the contrary, insufficient compression is caused and thereby density decreases to be white on the mediastinum portion.
Incidentally, when there is a big difference between signals in an image, the difference of signal values mentioned above is sometimes required to be kept as it is.
However, in the conventional dynamic range compression method wherein large increase correction is made when an unsharp mask signal is small, while, large decrease correction is made when an unsharp mask signal is large, there has been a problem that when compressing a high-density region, for example, the signal value in a directly exposed region which does not need to be corrected is lowered than is required, or the signal value which is extremely low in the non-irradiation region is forced to be raised in compression of a low-density region.
Further, when a portion where signal values are extremely different exists in an region that does not need compression, that portion can not be exempted from compression processing, which is a problem. For example, in compression processing of a low-density region in an image of a front view of a chest, when there is a metal such as a pacemaker or the like in a lung regions (a high-density region), there is caused a problem that the portion of the pacemaker shows a small signal value and thereby unwelcome compression is carried out when a correction value is established with a function of the signal value, resulting in approach between a signal level of the pacemaker and that of the lung regions, by which a difference of signal values can not be kept.
In the conventional compression method mentioned above, there has also been a problem that sufficient effects of dynamic range compression can not be obtained depending on a region because the frequency characteristic for obtaining an unsharp mask signal is constant.
Namely, frequency components contained in an image vary depending on a radiographing region, and there is also an individual difference even for the same region. Therefore, in the case of correction made with the unsharp mask signal obtained under the conditions of a mask type (a mask shape and a type of operation for unsharpening) and a mask size which are always constant, it has sometimes happened that sufficient effects of dynamic range compression can not be obtained depending on an object.
Further, even in the case of the same image, frequency components contained in the image vary depending on the region in the image. Therefore, the mask type and mask size which are constant sometimes prove to be improper.